Automobile probe-indicator. Free Technical Library article

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Automotive probe-indicator

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Encyclopedia of radio electronics and electrical engineering / Automobile. Electronic devices

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Various probes and indicators, despite the well-known error of the information obtained with their help, are in great demand among motorists. It is understandable - after all, these devices are cheap and available for self-production. In addition, their use does not require special training, and accurate measurements, as practice shows, are rarely needed.

This simple DC voltage tester is designed for troubleshooting electrical equipment and visual control of the voltage of the vehicle's on-board network. The device allows you to check the performance of the generator, voltage stabilizer, battery, to assess the degree of tension of the generator drive belt of the vehicle.

The mains voltage and the operability of electrical equipment are determined by the operation of the green and red LEDs installed in the probe. Its scheme is shown in Fig. one.

Resistors R2 and R5 are selected so that when the supply voltage is 14 V, the voltage at point A is less than at the zener diode VD1 by 1,5 V, and the "green" LED HL1 shines with half brightness. The voltage at point B exceeds the voltage at the Zener diode VD1 by 1.5 V, and the brightness of the "red" LED HL2 is also half the nominal. Both LEDs shine like this when the supply voltage is within 14 ± 0,5 V, which corresponds to the nominal mode of the on-board network of Zhiguli cars.

If the voltage in the network has increased to 14,6 V, the voltage at points A and B, respectively, increases compared to the voltage at the zener diode VD1, which leads to an increase in the brightness of the HL2 LED and the extinction of HL1. In the case of a decrease in the voltage in the network to 13,4 V, the HL1 LED, on the contrary, shines brighter, and HL2 goes out.

Thus, by the color and brightness of the glow of the LEDs, one can judge the mains voltage with a fairly high accuracy. The relative disadvantage of this indicator is some difficulty in establishing it due to the fact that a change in the resistance of one of the selection resistors leads to a change in the brightness of the glow of both LEDs simultaneously.

To adjust the probe, a voltage of 14V is applied to it. If at the same time the voltage at the zener diode VD1 is 5,6 V, by selecting resistor R2 the voltage at point A is set to 4,1 V and the HL1 LED is turned on. Then the resistor R5 is selected to a voltage of 7,1 V at point B and the HL2 LED is turned on. Change the supply voltage to 13,4 V, control the extinction of the HL2 LED. At a voltage of 14,6 V, the HL1 LED should go out.

The resistors in the probe can be any type with a power of 0,125 watts. It is better to use a miniature zener diode - KS156P KS409A, 2S156B.

Structurally, the probe is mounted on a narrow printed circuit board with subsequent pouring with epoxy compound in a collapsible cylindrical shape. The board drawing is shown in fig. 2. It is made of foil fiberglass 1 mm thick; shading in the figure shows sections of the foil. Output "+U_nm" is performed in the form of a rigid pointed probe, and the output "General." - a flexible conductor with a "crocodile" clip at the end.

After the compound has hardened, its excess is removed with a file. With this design of the probe, an additional case is not required.

Author: V. Dobrolyubov, Korolyov, Moscow Region; Publication: cxem.net

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